1. Field of the Invention
This invention relates generally to a hydraulic control system for an automatic gear change transmission. More particularly, the invention pertains to such a control system whose operation is controlled by an electronic control system which produces signals that cause selective pressurization of the gear change elements of the transmission.
2. Description of the Prior Art
Automatic transmissions for automotive use include planetary gearing in which the various gear elements are hydraulically braked in order to produce selected speed ratio ranged between the engine and drive wheels. Speed ratio changes result upon braking of a selected portion of the planetary gear drive.
Increasingly, automatic gear change transmissions are controlled by electronic means. Electronic control units operate reliably but it is possible that a cable may break or that the entire electronic control unit may fail. It is important, in the event of an electronic failure, that the various speed ratios of the transmission should be available to the operator in order to permit driving the vehicle to a service station for repair. It is known in the prior art that at least the first gear and the reverse gear of a transmission can operate in the event of an electronic system failure. U.S. Pat. No. 3,937,108 describes a system providing this function. U.S. Pat. No. 3,813,964 describes a control system wherein a valve prevents upshifting from the low speed range when the electronic control system fails and the transmission is set in the first speed ratio. It is preferable, however, that all the forward speed ratios and the reverse drive should be available to the vehicle operator if the electronic control system should fail. In this way, damage to the transmission can be avoided and the vehicle more easily driven to the service station.